Electrocardiograph (EKG) monitors are important, and non-invasive, diagnostic medical tools. An EKG waveform is a representation of some of the electrical activity produced by a beating heart during a period of time. Two or more electrodes are placed at various location on a patient's skin and connected to an EKG monitor. Electrical signals are generated in the heart. The signals are detected on the skin by the electrodes and received by the EKG monitor. The machine amplifies and processes the signals and converts them into a representation of the heart's activity, which may be analyzed and displayed as traces on a screen, printed onto paper, or both.
It is not the intent of this discussion to provide a detailed explanation of cardiology and the analysis of EKG traces. However, a general summary is useful for background purposes. FIG. 1 is an example of a strip chart of electrical signals from a 12-lead EKG monitor connected to a patient with a normal heart. FIG. 2 identifies individual waves, intervals, and segments. While there may be some confusion or ambiguity about the labeling of different “sections” of an EKG wave, for purposes of this application an “interval” contains one or more individual wave and a “segment” connects the end of one individual wave with the beginning of the next wave. Beginning from the left side of the chart in FIG. 2, the individual waves are: the P wave, the Q wave, the R wave, the S wave, the T wave, and the U wave (which may be overlapped and hidden by the T wave and the next P wave).
Beginning again from the left side of the chart in FIG. 2, the intervals are:                a. the PR interval, from the start of the P wave to the beginning of the QRS interval;        b. the PQ interval, which if used, is the same as the PR interval when the Q wave is present;        c. the QRS interval (also known as the QRS complex), which extends from the beginning of the Q wave to the end of the S wave;        d. the ST interval, extends from the end of the S wave to the beginning of the T wave; and        e. the QT interval, is measured from the beginning of the QRS interval to the end of the T wave; and        f. the RR interval, extends from the peak of one R wave of one beat to the peak of the next R wave of the next beat.        
The segments are:                a. PR segment, extends from the end of the P wave to the beginning of the Q wave;        b. ST segment, extends from the end of the S wave to the beginning of the T wave; and        c. TU segment, extends from the end of the T wave to the beginning of the U wave.        
FIG. 3 identifies the activity of the heart muscles during each of three major phases of a beat:                a. the P wave represents atrial depolarization;        b. the QRS interval represents ventricular depolarization; and        c. the T wave represents ventricular repolarization.The wave representing atrial repolarization typically occurs between the end of the P wave and the beginning of the T wave, but is typically hidden by ventricular activity.        
Many composite waveforms are interpreted or analyzed on the basis of received signals that are conditioned by a variety of processes to enhance the signal before the interpretation process begins. The conditioning process commonly includes amplification and filtering to remove from the received signal parts of the signal that are believed to not be useful for the purpose of the interpretation. One example of such a process is the interpretation of electrocardiograph (EKG) waveforms provided from EKG machines. An EKG waveform is a representation of some of the electrical activity produced by a beating heart during a period of time. The signals are detected by electrodes and received by the EKG machine, which processes the signals and converts them into a representation of the heart's activity.
It is not the intent of this discussion to provide a detailed explanation of cardiology and the analysis of EKG traces. However, it will be helpful to understand the relationship between PQRST notation and the related cardiac activity of atrial depolarization, atrial repolarization, ventricular depolarization, and ventricular repolarization. Generally, the P wave represents atrial depolarization, the QRS complex represents ventricular depolarization, and the T wave represents ventricular repolarization. Atrial repolarization (Ta wave) occurs during a time period beginning after the P wave and ending about the beginning of the T wave. The Ta is not often identified and is considered to be hidden by ventricular depolarization, which usually is much larger and occurs during about the same time period. The name Ta wave has been given to atrial repolarization activity even though its only obvious relation with the T wave seems to be that both represent repolarization. The subscript “a” is used to help avoid confusion with the T wave. It is known that atrial repolarization may influence an incorrect interpretation of the signal between the QRS complex and the T wave.
The problem of finding a Ta wave in a received EKG is somewhat like a sculptor's problem of finding an elephant in a block of stone. A problem reported to have been solved by skilled artists who say they simply remove everything that doesn't look like an elephant. Of course the problem of finding a Ta wave is different in at least two ways. On one hand, the problem is more complicated because one doesn't know what the hidden wave looks like, and the removed material must still be explained.
On the other hand, our material is more forgiving of mistakes and better suited to experimentation, analysis, automation, and inventive concepts.